There is a very large evergreen clematis starting to devour my detached garage. How far back can I cut this and when should I prune it back? It's one of the first early spring bloomers with white flowers, possibly armandii? But I am uncertain...I need help since I don't want to butcher it and lose it, but it needs a big haircut!

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Clematis armandii does have the reputation for taking over the world. According to the American Horticultural Society's Practical Guide on clematis (Clematis, by Charles Chesshire, 1999),
you can prune it AFTER is has finished flowering, which in Seattle, it normally does by the end of March. While this type of clematis can be pruned in late winter, it flowers on the previous year's wood, so pruning at that time may remove buds and prevent flowering that spring.

Step 1 - remove any dead, dying, damaged, or deranged shoots.
Step 2 - they suggest that no real pruning is necessary but you can cut it back to control its growth. But you do NOT want to cut it all the way back into old dark, woody growth.
Prune directly above a pair of strong side shoots.
Step 3 - you will need to keep after it each year to avoid a build up of tangled growth.

We are tearing out an old wood fence and replacing it with a new cedar fence, 6 feet high. We have a mature Clematis montana rubens growing on the old fence, prolific in growth and bloom, that I would very much like to save. Can I save it? We will have to start taking the old fence down right away. The new one is being installed next week, so I cannot wait until fall, which would probably be a better time to cut it back. Where do I begin pruning? Where do I stop? Anything I can do to lessen the shock to the plant?

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Clematis montanas are in pruning group 1 (or A) which means they do not take well to hard pruning. However, if it is the only way to save your Clematis, it is worth a try.

This is what the British Clematis Society recommends:
Category 1 (or A): No pruning.
"This category includes: C. montana If you wish to prune these types because they have outgrown their space they should be pruned immediately after flowering. You may or may not lose your plant as a result of the pruning. You might want to reduce the plant size over two or three seasons rather than in one go.
How-to: Start at the bottom of the plant and work your way up the stem to the first pair of plump, healthy buds. Prune the stem above the buds and remove everything above the cut. Treat each stem in a similar way."

Pruning is safer than transplanting:
"If a Clematis is to be replanted from an existing site, the late winter before bud break is the time to do this. However, it is only the large-flowered cultivars that generally can be replanted from an open ground position due to their large fleshy roots. The Clematis species and their cultivated forms have a very fibrous root system that usually breaks up when it is being dug up. The montana types are extremely difficult to replant once they have been established for more than two or three years." Source: The Gardener’s Guide to Growing Clematis, by R. Evison, 1998, p. 39).

I have a beautiful clematis in full bloom right now. Do I need to
dead head the spent blooms to make it bloom again? It is Clematis
Candida.

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In researching this question, I used Raymond Evison's
excellent 1998 book, The Gardener's Guide to Growing Clematis. Your
Clematis lanuginosa 'Candida' is considered a mid-season, large-flowered
type, and it often reblooms, according to Evison. In the chapter on
cultivation, he says, "Some clematis growers prefer to remove spent
flowerheads to encourage further crops of flowers, especially with the
early large-flowered single, double, and semi-double clematis. Certainly,
if the old flowers are removed with a length of stem with 2-3 nodes, new
growth will appear and a further crop of flowers will be produced. When
this is done, it is important to keep the clematis well watered and fed.
The only drawback...is that the attractive seedheads on this group of
clematis will be lost. A compromise can be achieved by removing only 50
per cent of the spent flowerheads..."

This link is also helpful (scroll down to
"Evergreen Vines" and look for appropriate height and light requirements).

You could grow bamboo, but I would recommend growing it in a container,
or a series of containers, as you do not want the roots to spread. I have
seen an effective bamboo screen between two houses growing in a long
rectangular lined wooden trough (lined with bamboo barrier). Some species
of bamboo are more tolerant of partial shade than others. Look for a
clumping, rather than a running, bamboo (like Fargesia) to be on the safe
side.

I have a client whose Clematis I just renovated. The client called to say the ends of the cut stems were frothing! What could be causing this?

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My best guess (and it is only a guess, since I am basing it solely on your description) is that it could be slime flux. According to the Royal Horticultural Society, "pruned stems may fail to reshoot and ooze a sticky substance--this is known as slime flux." To confirm this theory, ask the client if the frothy ooze was pinkish orange in hue, and if it had an unpleasant odor. Here is more about this problem from the Daily Telegraph's garden advice column by Helen Yemm excerpted below:
"This is not so much a disease as a condition that affects some trees and shrubs in spring. Bacteria enter the plant through cracks in the stem - which may well be caused by a combination of adverse weather conditions - and then attack the sap as the plant springs into action early in the year.
Slime flux is often fatal but it depends where the damage is. Everything above the oozing wound will certainly die and should be cut down. However, the plant may well shoot out from below and recover.
Slime flux will not spread to other plants as diseases do, nor will it contaminate the soil like clematis wilt. However, its occurrence would indicate that, for some reason, the general growing conditions in that part of the garden have become unsuitable. If you do lose your clematis, then perhaps it would be a better idea to plant a new one in a less exposed spot."

I'm planning an arbor in my garden which will be 6' wide, 4' deep, and 6-7' high. I'd like to plant on one side Clematis 'Polish Spirit' . Can you suggest a climber that would go with the Clematis for the other side? The other climber would be in partial shade. Great Plant Picks says that Clematis, with pruning, can be managed to 12' long. How wide can it get or, a different question, would I need to plant one or two for coverage of the planned arbor? Also if you have other favorite arbor climbers and combinations of climbers I would love to know about them.

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There are many choices for combining with your Clematis. Climbing roses are considered a natural companion of clematis and, depending on your color preferences, perhaps something which contrasts with the deep purple would be nice: either white or a pale pink. When you say the other side is in part shade, I wonder how many hours of sunlight it does get. Once the prospective climber reaches the top, will it be in sunlight? You could also plant a clematis of contrasting color or flowering season on that side. There are many books in our library on combining clematis with other plants, such as Companions to Clematis by Marigold Badcock (Master Craftsman, 2000), and Clematis as Companion Plants by Barry Fretwell (Cassell, 1994). There is even an entire book on clematis and roses: The Rose and Clematis as Good Companions by John Howells (Garden Art Press, 1996).

If you like roses but not thorns, you could grow a Lady Banks rose which is thornless, but you would have to keep it pruned (it wants to be 20 feet high). The Northwest-based gardening website, Paghat's Garden, describes it.

Rosa 'New Dawn' is a very popular choice in the Northwest, and is recommended by virtually every local garden writer I have come across. Here is a (non-local!) description from Missouri Botanical Garden.

Another option might be the white variety of potato vine, Solanum laxum 'Album,' though it may be a bit tender in cold winters.

Eventually your clematis will cover the width of the arbor, but Clematis viticella cultivars like yours are easy to prune (late winter/early spring) and keep to a manageable size. If you like the idea of having more than one color/plant/flowering season, it's perfectly fine to add another climber. I would avoid anything that is highly aggressive or rampant (Passiflora, Akebia, Campsis, Clematis montana, and others), as you don't want to overpower 'Polish Spirit.'